You just need to ride in a bunch of saddles to see what type of seat and size of seat you like best. It's personal preference.
For the horse, take a wither tracing.
- Use a 15" flexi-curve from an arts & crafts store and take the shape of his back, about 2" behind the back edge of his shoulder blades. Take your time and really get it to fit to the curves of the muscles.
- Carefully lift it off and place it on a large piece of cardboard. Make sure you place it so you know which was the left and right side of the horse.
- Very carefully trace the inside contour of the flexi-curve, keeping your pencil mark as close to the flexi-curve as possible. Mark L and R "inside" the horse shape.
- Cut out your horse shape with an exacto knife or very sharp (fresh blade) box cutter. Again, being very careful to maintain the shape.
Now, take your cardboard "horse" with you to the tack shop. Put the cutout under the saddle, at the front edge of the conchos. Pay attention to the L and R markings on your "horse." The saddle and cutout should have the same angle. The top of the cutout should have about 2-2.5" of space between it and the underside of the pommel/swell.
There is more to saddle fitting than the front of the saddle, but the cardboard cutout will help narrow down your choices. (take your soft measuring tape with you to the tack store!)
Two measurements you can take:
- From the back edge of the horse's shoulder blades to the mid-line of your horse's flanks. You don't want your saddle skirts to be able longer than that, front to back. It's preferable to have a saddle smaller than that measurement.
- From 2" behind the shoulder blades to the last rib. You want your saddle to be 1" smaller or less that this measurement from the front edge of the front concho to the back edge of the back concho.
When you get your saddle home, hopefully on "trial," you can try it on your horse to see if you picked one that really fit. I can give you instructions on how to check for fit.